Lithium secondary battery, recently used in an increased amount, mainly uses Li-containing cobalt oxide (LiCoO2) as a positive electrode active material. Additionally, the use of Li-containing manganese oxide, such as, LiMnO2 with layer crystal structure, and LiMn2O4 with a spinel crystal structure, and a Li-containing nickel oxide LiNiO2, is also considered.
The Li-containing cobalt oxide (LiCoO2) of the positive electrode active materials is currently widely used because of excellent overall properties such as superior cycle characteristic, and so on. However, the Li-containing cobalt oxide has several problems such as relatively high price, a low charging and discharging current amount, which is about 150 mAh/g, unstable crystal structure at 4.3 V of voltage or higher, risk of fire from reaction with an electrolyte, and so on.
In order to solve the problems, suggestions have been made, which include the technology for coating an outer surface of the Li-containing cobalt oxide (LiCoO2) with a metal (e.g., aluminum) so as to allow operation at a high voltage, technology for thermally treating Li-containing cobalt oxide (LiCoO2) or mixing with another material, and so on. However, a secondary battery composed of such positive material may show weak stability at a high voltage or may have limited application to a mass production process.
Because the lithium manganese oxide such as LiMnO2 or LiMn2O4 has advantages of using the eco-friendly manganese which is plentiful as a raw material, it gathers many attentions as a positive electrode active material that can replace LiCoO2, but the lithium manganese oxide has disadvantages such as small capacity and bad cycle characteristic.
The lithium nickel based oxide such as LiNiO2 costs less than the cobalt-based oxide, while it shows a high discharge capacity when charged at 4.3 V. Accordingly, a reversible capacity of the doped LiNiO2 may approach to about 200 mAh/g which exceeds a capacity of LiNiO2 (about 165 mAh/g). However, the LiNiO2-based oxide has problems of rapid phase transition of a crystal structure according to a volume change accompanied with charge/discharge cycle, and generating of an excess gases during cycle.
In order to solve the above problem, there is suggested a lithium transition metal oxide, which is in a form in which a portion of nickel is substituted with another transition metal such as manganese, cobalt, and so on. The nickel-based lithium transition metal oxide substituted with metal has advantages of relatively excellent cycle characteristic and capacity characteristic. However, the cycle characteristic is rapidly lowered when used for a long time, and stability problem occurring from storing at a high temperature is not yet solved.
Further, as the recent mobile device gradually becomes high-functioned to provide various functions while being light-weighted and miniaturized continuously, and as attentions are received on the secondary battery as dynamic power resource of an electrical vehicle EV and a hybrid electrical vehicle HEV, which are suggested as methods for solving the air pollution of a related gasoline vehicle and a diesel vehicle which use fossil fuel, use of the secondary battery is expected to further increase. Given this, attentions are growing for not only the above problems, but also the problems of battery stability and high-temperature storing characteristic at a state of high level capacity and high electrical potential.
Accordingly, a new technology for simultaneously solving problems of output and service life characteristics is highly requested.